A cellulose-derived supramolecule for fast ion transport

Supramolecular frameworks have been widely synthesized for ion transport applications. However, conventional approaches of constructing ion transport pathways in supramolecular frameworks typically require complex processes and display poor scalability, high cost, and limited sustainability. Here, we report the scalable and cost-effective synthesis of an ion-conducting (e.g., Na+) cellulose-derived supramolecule (Na-CS) that features a three-dimensional, hierarchical, and crystalline structure composed of massively aligned, one-dimensional, and ångström-scale open channels. Using wood-based Na-CS as a model material, we achieve high ionic conductivities (e.g., 0.23 S/cm in 20 wt% NaOH at 25 °C) even with a highly dense microstructure, in stark contrast to conventional membranes that typically rely on large pores (e.g., submicrometers to a few micrometers) to obtain comparable ionic conductivities. This synthesis approach can be universally applied to a variety of cellulose materials beyond wood, including cotton textiles, fibers, paper, and ink, which suggests excellent potential for a number of applications such as ion-conductive membranes, ionic cables, and ionotronic devices.

The color evolution of cellulose paper over 14 days of soaking in the Cu 2+ -saturated NaOH solution. Paper is used here as a model cellulose material to clearly show the color change.

Supporting Discussion I: Structural analysis
We built models to analyze the structure of the Na-CS by positioning atoms within the unit cell in reasonable ways until we found the best match of the calculated XRD patterns with the experimental data. The cellulose chain conformation is restricted by the AGU rigidity. As a result, only limited combinations of AGU-copper complex positions are permitted within the constraint of the unit cell. We built models for all combinations of the permitted handedness, direction of chains, and position of copper ion, calculated the theoretical fiber diffraction pattern of each possible configuration using a computer program we wrote, cross checked with XPolar and Mercury software, and selected the model that fit the XRD and XANES data the best. The modeled Na-CS structure can be achieved by simply rotating the cellulose chains in the Na Cellulose II structure reported by Atkins et al. (29) by 60˚ around the center of the chain in the z direction, then spreading out the rotated molecular chains with enough space, and finally inserting a copper ion.
The model with the copper ion coordinating with the O2 and O3 atoms of neighboring chains was built with a 3-fold symmetry operation reported in the literature (29,72) with slight numerical modification to the bond angles between the neighboring AGU rings along the chain into an accepted range. To semi-quantitatively analyze the quality of the model, a calculation of the 1D XRD pattern based on the P3221 space group in TOPAS with consideration of preferred orientation of the cellulose fiber is given in Fig. S12. The used CIF files are included in the Supporting Information. The dihedral angles at the glycosidic linkage are Φ = -79.45˚ (C5-C1-O1-C4') and ψ = 149.81˚ (C1-O1-C4'-C3'), these two torsion angles are not directly comparable to the native cellulose conformation with a twofold axis. The C1-O4'-C4' bond angle is 116.38˚, the O5-C1-O4' bond angle is 108.43˚, the C2-C1-O4' bond angle is 105.03˚, and the bond length between C1 and O4' is 1.421 Å. All bond angles and bond lengths are within normal bond angle and length ranges and compared in Table S2. O6 is set to the gg conformation in the current model with χ~60˚ (O5-C5-C6-O6) (29). Note that the model based on P3221 symmetry has cellulose chains arranged in anti-parallel packing. A parallel packing model based on P62 symmetry is also possible yet less likely (Fig. S11). The XRD data could not distinguish between the P3221 and P62 symmetries.
However, the materials before transforming to Na-CS in this work typically displays an antiparallel packed Na Cellulose II crystal structure, therefore it can be assumed the anti-parallel packing P3221 model is more probable, as transformation from anti-parallel packing to parallel packing has not been observed in cellulose crystals.
The next part of the modeling study was to find the position of the copper ion. Copper ion coordination with the O2 and O3 of atoms of the cellulose AGU structure has also been reported for the dissolution of cellulose in amine-copper-ion complex aqueous solution (73). It has been found that the copper ions bonding to O2 and O3 is indeed the most reasonable scenario (74)(75)(76).
To analyze the size of the elementary fibril of the Na-CS, a wood-based Na-CS sample was soaked in DMSO to exchange and remove NaOH and water, followed by drying in vacuum under room temperature. The Na-CS structure was preserved after the treatment.      The molecular structure evolution. The molecular structure evolution from Cellulose Iβ to Na Cellulose I, then to Na Cellulose II, which are driven by NaOH, and finally to Na-CS upon the slow copper insertion (29,72).   Table S2), the authors deemed the structure satisfactory. It should be noted that as the Na-CS is strongly affected by both copper and sodium ions, a larger scale calculation or simulation including all the factors is still needed to fully understand the structure.
Fig. S12 1D XRD pattern calculation for the Na-CS structure with preferred orientation considered. The experimentally measured XRD data ( Figure 2D, Figure S7) is shown in blue, and the calculated diffraction 1D pattern produced by TOPAS (with preferred orientation (001) considered from the P3221 model with water included) is shown in red. The experimental and calculated data show the same trend, indicating the model captures the essence of the structure. The difference between the experimental data and calculation may derive from the position of water and Na + as well as the cellulose chain arrangement.

Fig. S13 2D fiber XRD pattern of Na-CS with an azimuthal integration around the (100)
peak. The 2D fiber XRD was conducted on a Xenocs Zeuss SAXS system with a copper Kα microfocusing source. The sample was sealed in an X-ray capillary and scanned in vacuum. Only 1 quadrant was scanned.

Fig. S14 1D fiber XRD pattern from Na-CS with an azimuthal integration around the (100)
peak and a Gaussian peak fitting on the (100) peak as shown in Fig. S13. The fiber diameter can be estimated based on the Scherrer equation as 100 Å, with the shape factor assumed to be 1 and full width at half maximum assumed to be 0.0155 rad from the fitting. The fibril diameter (100 Å) is larger than the reported diameter of the cellulose elementary fiber (20-30 Å), the reason is likely due to the swelling of the nanofibre during copper intercalation, plus the effect of two or more Cellulose I elementary fibrils merging to form anti-parallel chain packing during the NaOH solution treatment.

Fig. S15 Representative Nyquist plots of the non-pressed and pressed wood-based Na-CS in 20 wt% NaOH at 25 ℃.
Due to the complex microstructure of the wood-based Na-CS, the conductivity was extrapolated based on the following equation: Rmeasured = Rbulk + Rcontact = L/σS + Rcontact, where R is the resistance, L is the length, S is the surface area, and σ is the conductivity (see Methods for details).    Table S3. Comparison of porosity between pressed wood-based Na-CS and glass fiber membrane.

Supporting Discussion II: Modeling Na + transport in Na-CS
Born-Oppenheimer molecular dynamics simulations were performed to explore the diffusion mechanism of NaOH4.5H2O and NaOH9H2O aqueous solutions inside the Na-CS at elevated temperature (120 ºC). The starting composition of Na-CS considered was fully deprotonated on the O2, O3, and O6 sites for NaOH4.5H2O and fully protonated for NaOH9H2O (see Methods for details). Because of the computational expense to generate trajectories of ~30-50 ps for these systems, several initial geometries (what we call replicates) are extracted from a series of force field simulations.
The diffusion of Na + ions in the Na-CS is highly directional in the direction parallel to the channel. However, we observe that 1D mean square displacements (MSDs) in this parallel direction averaged over the 28 Na + ions per trajectory are quite small (< 1 Å) (72) over the linear regime. From the slopes of the MSD vs. time curves in the time range of 5 ps to 15 ps, the diffusion coefficient of Na + ion in Na-CS is estimated to be 2 ± 0.4 10 -10 m 2 /s, which translates to a maximum (ideal) conductivity due to Na + on the order of ~45 mS/cm, despite the use of revPBE to improve the dynamic properties of water. For the sake of brevity and because the NaOH9H2O trajectories show no appreciable difference from the NaOH4.5H2O trajectories, we do not show average MSDs or per particle MSDs for the NaOH9H2O trajectories. It may also be the case that OPLS/AA is too aggregating even with scaled charges and the timescale of the DFT simulation even at high temperature is not enough to see dissociation of the ion pairs. Related to the NaOH4.5H2O simulations, one unique aspect DFT simulation provides over the force-field-based dynamics simulations, however, is the explicit treatment of the structural diffusion mechanism of OHin solution through the H3O2intermediate described by Tuckerman et al. (81,82). Several of these exchanges were observed to occur in the solvation shell of the Na + ions immobilized on the cellulose wall, forming the octahedral complex RCONa(H2O)4OH. In instances where the coordinated OHwas converted to H2O, the resulting Na + (H2O)5 species is seen to quickly dissociate from the cellulose. This would suggest that the structural diffusion mechanism of the solvated OHplays a role in the exchange of cations between the nano-confined solution phase and the cellulose/electrolyte interface. It is worth noting also that the Cu 2+ ions remained in place over the length of the simulation further confirming stability of the proposed Na-CS model. This plot demonstrates the directionality of the diffusion pathway (i.e., from one cellulose site to another) and the importance of Na + ion exchange between the cellulose/electrolyte interface and the solution phase in the middle of the channel. By examining the per particle MSD (all 112 ions from all 4 trajectories, Fig. S20), we observed significant heterogeneity of Na + displacements on the simulation timescale. The same is true for the case with the NaOH9H2O as well, most cations are immobilized (again, relative to DFT timescales) on the anionic sites along the cellulose wall.
Fig. S20 examines the correlation between the Na + coordination shell composition (x-axis) and the Na + 's proximity to the cellulose framework, namely the per particle MSD computed to 10 ps (yaxis). The probability that the ion is <3.2 Å away from any of the cellulose oxygen atoms over the length of the simulation is demonstrated using a blue-to-red color gradient. From this figure it is seen that the fastest moving ions tend to have higher liquid phase coordination numbers and do not spend as much time near one of the anionic sites on the cellulose (red dots, upper far right of figure). However, as is demonstrated in Fig. S19, nano-confinement of Na + ions in the cellulose channels tends to lead to a more directional diffusion pathway parallel to the channel when large hopping events do occur.  Born-Oppenheimer molecular dynamics modeling of the Na + transport in Na-CS, relating diffusion to the coordination environment of the Na + ions. (A) The per particle MSD computed to 10 ps (y-axis) against the average coordination number of Na + to water and/or OH -(x-axis) and probability that the ion is <3.2 Å away from any of the cellulose oxygen atoms over the length of the simulation (color). Blue indicates a high probability to sit <3.2 Å from cellulose oxygen, while red indicates a high probability to reside >3.2 Å from cellulose oxygen atoms. (B) The Na + ions near and interacting with cellulose oxygen diffuse slower (left), while those at the channel center, with higher liquid phase coordination numbers and that do not interact strongly with the cellulose molecular chain, diffuse faster (right).   No discernable difference of the microstructures can be observed between the pristine and Na-CS filter papers.